Everything above is correct. The 365SP 371/372 are all essentially the same saw, with different P/C and carbs. HD air filter is/was an option but most dealers ordered the saws with the HD setup. Keep in mind that the new saws you see are most likely XT versions, no big deal the case is the same. In fact if you order a new case for a 372XP as a spare part you will get the XT case. The 371 P/C you have will work on any of the cases. One additional note, the XT saws make a little more torque than the XP versions, and a 365XT can be converted to a 372XT in less than an hour, you grind the vane out of the transfer port covers and viola, you have a 372XT!